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Site U13491

Expedition 324 Scientists2

Background and objectives


Integrated Ocean Drilling Program Site U1349 (proposed Site SRCH-5) on Ori Massif was the fourth site completed during Expedition 324. This site is near the center of a north–south transect across Shatsky Rise and was intended to sample the summit of the large Ori Massif volcanic edifice (Fig. F1). Only one hole, U1349A, was cored at the site. According to the initial expedition plan, Site U1349 was to be the second deepest penetration site to be drilled, with a target of ~200 m into igneous basement. As with all Expedition 324 sites, the penetration rate of drilling was slower than expected, and this goal was not reached. Unlike some other sites of this expedition, Hole U1349A was terminated because the material being recovered did not justify further drilling. Approximately 86 m of basement rocks was recovered, but alteration was severe and did not appear to improve with depth. Thus, the science party decided to cease drilling and to move on to Site U1350. Despite the alteration, the rocks recovered from Site U1349 tell a story of eruption of primitive lavas near or above sea level, erosion, and extensive and variable alteration with burial and submergence.

Ori Massif is the second largest volcanic construct within Shatsky Rise, with a volume of ~0.7 x 106 km3 (Sager et al., 1999). Similar to Tamu Massif, Ori Massif may have formed over a geologically short period of time (<1 m.y.) with a high effusion rate, but the actual age and duration are unknown. Because Ori Massif is separated from Tamu Massif by a narrow, rectangular, faulted basin ("Helios Basin;" see F2 in the "Expedition 324 summary" chapter), there has been speculation that Ori and Tamu massifs formed together and later rifted (Sager et al., 1999; Nakanishi et al., 1999). In the context of the plume head hypothesis, Ori Massif appears to represent the eruptions during a transition in volume from plume head (Tamu Massif) to plume tail (Papanin Ridge).

Magnetic lineations surrounding Ori Massif imply it formed on lithosphere of earliest Cretaceous age. Anomaly M16 brackets the southeast flank, whereas Anomaly M14 crosses the northwest flank (Nakanishi et al., 1999; see F3 in the "Expedition 324 summary" chapter). According to the geomagnetic polarity timescale (GPTS; Ogg et al., 2008), the age of the lithosphere at Ori Massif is ~142–140 Ma, which may also be the age for Ori Massif if it formed nearly synchronously with the lithosphere, as implied by compensation (Sandwell and McKenzie, 1989). The northeast and southwest flanks of Ori Massif appear to be bounded by fracture zones (Nakanishi et al., 1999), and this bracketing by spreading ridges and fracture zones, which are orthogonal, may be the reason that the massif appears subcircular or nearly square in plan view (Sager et al., 1999).

Like Tamu Massif, Ori Massif has the appearance of a large central volcano with low flank slopes (Sager et al., 1999). Too few bathymetry data exist to say which flanks of this edifice are smooth and which are faulted, although the shape of the southern margin implies rifting. As with other Shatsky Rise massifs, the rise top is covered with pelagic sediment that reaches maximum thicknesses of 600–700 m on existing seismic profiles (Figs. F2, F3) (Sager et al., 1999; Klaus and Sager, 2002). Like Tamu Massif, Ori Massif has volcanic summit ridges, although all are buried, or mostly so, and therefore are not as prominent as the large ridge (Toronto Ridge; Sager et al., 1999) atop Tamu Massif. The origin of these ridges is not known; however, they appear to be volcanic constructs and not fault-bounded ridges (i.e., horsts). Site U1349 was drilled into one such ridge, a flat-topped ridge located near the middle of the Ori Massif summit (see fig. 10 in Sager et al., 1999, for a seismic line crossing near the site). It was thought that because this ridge has a flat top and is at the tallest point on the volcano, it was probably eroded flat by wave action at sea level. Thus, evidence for subaerial exposure was an expected result from Site U1349 drilling.

Scientific objectives

Sampling the summit of Ori Massif was an important objective because this volcano is a major edifice within Shatsky Rise. Furthermore, Site U1349 is at the center of a transect of sites along the axis of Shatsky Rise expected to yield age and geochemical trends within the plateau. As with most Expedition 324 sites, the operational goal for the site was to drill through the sediment overburden, core the oldest sediment overlying igneous basement, and core as deeply into the igneous formation as possible in the time allowed.

Scientific objectives of all Expedition 324 sites are similar (for more details and rationale, see the "Expedition 324 summary" chapter). Coring of igneous rock was planned to determine the age of igneous basement so that the age progression and duration of volcanism at Shatsky Rise can be constrained. A critical objective at Site U1349, and indeed at all Expedition 324 sites, was to core enough igneous rock of suitable freshness and composition to allow at least one reliable radiometric date to be established. Igneous rocks are critical to geochemical and isotopic studies, the goals of which are to establish the elemental compositions of the rocks, variations in compositions, and the isotopic characteristics. Such data are crucial for determining the source of magma, inferring its temperature and depth of melting and crystallization, and deducing the degree of partial melting, as well as tracking its evolution with time. Operationally, this meant that at Site U1349 the goal was to core a representative suite of igneous units that were fresh enough to provide reliable geochemical and isotopic measurements.

Expedition 324 also sought to constrain the evolution of Shatsky Rise by collecting samples for a host of nongeochemical studies that focused on varied aspects of rise geology. Physical volcanologists, structural geologists, and logging geophysicists will use cores and logs to infer the eruption style, igneous products, and physical structure of Shatsky Rise. Given its size and inferred eruption rate, Ori Massif is likely to be an example of an unusual volcanic construct, the development which is poorly understood. Shatsky Rise core samples will also be used to study the submarine alteration of igneous rock and its effect on other analyses. Considering the altered nature of the rocks recovered from this site, alteration studies will play a critical role in better understanding and using other data sets. Studies of sediments overlying igneous basement are planned to better understand the paleontological age of Shatsky Rise sediments and the processes and rates of Cretaceous sedimentation atop the rise volcanoes. It was hoped that shallow-water sediments would be cored at Site U1349, but sediment recovery was poor. Nevertheless, sedimentologists will carefully examine the sparse sediments that were recovered. Sediment types and paleontological environment data will hopefully indicate the paleodepths of sediment deposition, which is important for understanding the eruption and subsidence history of the volcanic edifices. Paleomagnetic study of the samples recovered during Expedition 324 seeks to determine the magnetic polarity of basement for comparison with surrounding magnetic lineations and the GPTS, as well as the paleolatitude of the rise and its plate tectonic drift. Physical properties of Shatsky Rise core samples will be measured to better understand the nature of the rocks that make up the rise and to constrain fundamental physical properties that affect geophysical imaging and remote sensing. Because of the extensive alteration of Site U1349 rocks, physical property measurements will be important for determining changes in rock properties with alteration.

1Expedition 324 Scientists, 2010. Site U1349. In Sager, W.W., Sano, T., Geldmacher, J., and the Expedition 324 Scientists, Proc. IODP, 324: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.324.106.2010

2Expedition 324 Scientists' addresses.

Publication: 3 November 2010
MS 324-106